The central goal of the research here is to understand how the vertebrate brain is able to produce an enormous repertoire of vocal communication signals. The brain circuitry underlying vocalization is highly conserved across vertebrates, including teleost fish. This work uses midshipman fish as a model system to examine, for the first time, vocal motor patterning in midbrain vocal circuits and the modulation of such patterning by the neuropeptide arginine-vasotocin (AVT). AVT is potently involved in regulating social behavior across species. Midshipman, in turn, are wholly dependent on social interactions mediated by vocal communication for successful reproduction. Precisely regulated vocal output is critical for proper communication, in this species and many others, including ourselves. Here, extra- and intracellular recordings will be used to characterize relationships between the firing patterns of single midbrain neurons mad vocal output. AVT and AVT antagonists delivered locally within the brain modulate evoked vocal output; recordings of midbrain neuronal activity will delineate how modulation of such activity by AVT effects changes in vocal output. Finally, AVT and AVT antagonists will be delivered to vocal brain areas in behaving fish to determine precisely how AVT modulates natural vocalizations. Because the midbrain circuits studied here are conserved across vertebrate species, as is also AVT, this work promises to inform a larger understanding of neural mechanisms of vocal communication. [unreadable] [unreadable]